In a commercially competitive environment, airlines are at their most vulnerable if another airline starts up on the same route. Although the competitor may not get a great percentage of the business it ruins the profitability margin. The best way to prevent this is to fill your main hubs and routes up with flights so much that no competitor can mount a major challenge. RJs represent the cheapest way of doing this. Larger aircraft at lesser frequency are a much more economic way to do things, but this is no benefit if another airline moves in and takes part of your traffic.

Here in London, Heathrow airport is effectively full, determined by runway capacity, and has been for many years (there have been some improvemnets which have increased the runway rates, allowing extra flights). If they were miraculously to get double the runways and capacity tomorrow, British Airways would move astoundingly quickly to fill up those extra slots as well before anyone else got them. They would have to. Otherwise they would lose their competitive advantage of being the dominant airline.

There has also been a great expansion of flights away from the old core routes, such as the New York to LAX example. This flight a generation ago probably had passengers heading for San Diego, Honolulu etc who now fly direct or through a midwest hub, as well as transatlantic passengers heading for LA who now go by nonstop flight.

BTW Delta did not serve this route in 1974. The airlines were the old stalwarts American, United and TWA. Delta's 747s were on routes like Atlanta to LAX.